Capacitors are one of the most fundamental electronic components (circuit elements) in an electronic circuit system, and are widely used in various electronic devices. Use of a computer simulation for precise analysis and design of electronic circuits can eliminate the time required for prototype work and experiments and can therefore greatly reduce the design cycle time of electronic devices. To do so, however, a highly accurate wide-band equivalent circuit model of the electronic component is required. When a circuit including a capacitor is to be designed using a SPICE simulator, for example, a SPICE model that can accurately indicate characteristics of the capacitor is required.
Due to the presence of various parasitic parameters, frequency characteristics of the impedance of a capacitor and more particularly the frequency characteristics in a high-frequency band become considerably complicated. This makes it very difficult to obtain a highly accurate SPICE model. Specifically, when a DC (direct current) bias is applied to a capacitor (Class2), the characteristics of capacitance and impedance change significantly. Therefore, if circuit analysis or design is performed by using an equivalent circuit with a nominal capacitance value or without a DC bias, it is inevitable that the results thereof greatly differ from the actual circuit characteristics, which may lead to a complete failure of the design of the electronic device. From this perspective, it is important to construct a SPICE model for a capacitor (Class2) with a DC bias applied thereto that can be suitably used for detailed analysis and design of a circuit, and to establish analysis and design methods utilizing the model.
The following Non-Patent Documents, for example, disclose such equivalent circuits for a capacitor. First, Non-Patent Document 1 discloses equivalent circuits for a multi-layer chip capacitor (MLCC) and a tantalum electrolytic capacitor by John D. Prymak. FIG. 18A shows one of such circuits. In this equivalent circuit, the characteristics changes due to frequencies are taken into account only for an equivalent series resistance ESR. Although other elements are also affected by frequencies, the characteristics changes are not taken into account. Further, the characteristics changes due to frequencies in the equivalent series resistance ESR are calculated by estimate equations that lack a valid theoretical basis. With regard to changes in circuit elements caused by the DC bias, only the equivalent series resistance ESR and a capacitance element Cnom are taken into account, and the characteristics changes in the circuit elements caused by the DC bias are calculated by estimate equations that lack a valid theoretical basis.
Non-Patent Document 2 describes an equivalent circuit disclosed by Brian Hirasuna and Coby Bassett. Here, a SPICE model of a nonlinear capacitor is configured by employing an idealized C circuit model based on an ABM (Analog Behavioral Modeling) function of a SPICE simulator. As shown in FIG. 18B, the capacitor is replaced with a voltage dependent current source. An output current “i” of the voltage dependent current source is expressed by a product of the time derivative of a voltage applied across the capacitor and the capacitance as shown in the figure. A change in the capacitance caused by the voltage is represented by a table (voltage-capacitance) or a polynomial.